The present invention relates to package decorating apparatus and, more particularly, to high speed can printing machines having printing cylinder assemblies comprising drive shaft mounted printing cylinders with attached printing plates which are used to transfer ink images to a blanket means which in turn transfers the ink images to cans.
Can printing (decorating) machines, especially high speed continuous can printing machines, operate by the impingement of a rotating, image-carrying blanket wheel and an oppositely rotating can carrying mandrel wheel. The blanket wheel comprises a blanket which is at least as wide as the length of the cans being printed. The blanket carries a series of wet ink images circumferentially spaced on its resilient periphery. The mandrel wheel carries a series of circumferentially spaced, rotatable shaft, or mandrel, assemblies, over which cans are fitted. The cans rotate on the mandrel wheel into registry and contact with the images and surface of the blanket wheel. Each mandrel generally includes structure for removing cans from or drawing cans onto the mandrel shaft.
The wet ink images carried by the blanket wheel are transferred thereto by one or more printing cylinder assemblies, each color of the ink image being transferred to the blanket wheel by a separate assembly.
Each printing cylinder assembly includes a rigid, high strength printing cylinder, mounted on a central cylinder drive shaft which is positioned in parallel alignment with the blanket wheel axis of rotation. The surface of the printing cylinder is smooth, generally highly polished metal. In most existing can printing machines a printing plate having a width substantially equal to the axial length of the printing cylinder and having a length substantially equal to the circumference of the cylinder is attached about the circumference of the printing cylinder.
Printing plates of the type presently used in high speed can printing machines are generally laminates comprising an inner layer of high strength material such as steel which is positioned in contacting engagement with the exterior surface of the printing cylinder, and an outer layer generally comprises a non-metal material such as plastic which is bonded to the surface of the inner layer of material. The outer layer of the printing plate has a raised pattern portion having the same configuration as the image to be printed on a can. The pattern is provided by chemical etching or other methods which are well known in the art. The pattern portion is coated with ink by conventional inking apparatus such as ink rollers. The ink carried on the pattern is then transferred to the blanket wheel by rolling contact between the blanket wheel and the printing plate, thus producing an ink image on the blanket wheel having the mirror image configuration of the raised pattern portion of the printing plate. The image on the blanket wheel is transferred to a can to be printed by subsequent rolling contact of the can with the blanket wheel.
High speed decorating systems are described in Stirbis U.S. Pat. No. 4,267,771, issued May 19, 1981; Sirvet, U.S. Pat. No. 4,037,530, issued July 29, 1977; and McMillin et al U.S. Pat. No. 4,138,941, issued Feb. 13, 1979, all of which are hereby incorporated by reference.
In earlier machines, in the type of system described in Stirbis, U.S. Pat. No. 4,267,771, printing cylinders were provided having a circumference substantially equal to the circumference of a can to be printed plus the length of the space to be allowed between images on the blanket wheel.
As the operating speeds of can decorating machines have increased with newer technology, the rotational speeds of printing cylinders used thereon have increased proportionally and have become a source of problems because of inertial effects associated with rapid rotation. One method of dealing with such rotational speed-related problems has been to increase the circumference of the printing cylinder and to provide multiple image patterns on an associated printing plate rather than a single image pattern. For example a printing cylinder assembly having a circumference substantially equal to twice the circumference of a can to be printed plus twice the length of the space between images on the blanket wheel could be provided with two symmetrically spaced apart image patterns rather than one. Such a printing cylinder assembly would rotate at half the speed which would be required of a single image cylinder assembly operating at the same machine speed. However, it has been discovered that use of a larger diameter printing cylinder with a multiple image pattern printing plate, under certain conditions, introduces mounting accuracy problems between the printing plate and printing cylinder. For example, in printing cans such as beverage cans having a diameter of substantially 2.5 inches, it has been discovered that for reasons which are probably related to surface friction between the printing cylinder and the double image printing plate, accurate mounting of a double image printing plate was impossible. Any attempted mounting of the double image printing plate causes deformation of the plate placing the images thereon slightly out of phase or causing image distortion in at least one of the image patterns. Surface lubrication of the printing cylinder, surprisingly, does not alter these results.
It has also been found that when printing cylinders of sufficient size to carry two can-image patterns on the surface thereof are used, accurate mounting of the cylinder on a drive shaft becomes problematic. Due to the larger diameter of the cylinder, any inaccuracy produced by inexact tolerances between the cylinder bore and the shaft received therein are magnified and tend to produce "wobble" resulting in misregistry and deformation of the ink patterns printed on the blanket wheel surface. However, if extremely accurate tolerances with minimal clearances between the shaft and cylinder bore are provided, it becomes very difficult and time consuming to mount or remove the cylinder from the shaft. Since cylinder plates wear and must frequently be replaced, it is necessary to remove and remount the cylinder periodically. Thus, close tolerance mounting has, in the past, not been considered practical.
It would be desirable to produce a can decorating machine capable of operation at high speeds without encountering printing cylinder assembly problems related to rapid rotational speeds. It would also be desirable to produce such a can decorating machine which does not cause distortion of the printed images when using printing cylinder assemblies having multiple image patterns. It would also be desirable to provide apparatus for quickly and accurately mounting a printing cylinder on an associated drive shaft.